US 2013/0030176 A1 discloses of method for preparation of 4,6-dichloro-5-nitro-2-(propylthio)pyrimidine and its use as intermediate for the preparation of ticagrelor. Example 10 discloses a yield of 233.5 g of the 4,6-dichloro-5-nitro-2-(propylthio)pyrimidine (MW 266 g/mol) with a purity of 99.45% is disclosed; with a MW of 229 g/mol for the substrate 5-nitro-2-propylthiopyrimidine-4,6-diol and a starting amount of 200 g the yield is 99.9%, which is admittedly very high.
Nevertheless the method uses phosphorous oxychloride (MW 153 g/mol) as Cl source for the exchange of the OH residues against Cl. The given 425.6 g of phosphorous oxychloride are converted by the reaction ultimately to 271 g phosphoric acid H3PO4 (MW 100). So 1.16 times, of the weight of the product, of phosphoric acid needs to be disposed of. This waste is a serious environmental challenge as well as a cost factor.
There was a need for a process that does not pose the mentioned waste problem in form of H3PO4.
An extensive screening of phosgene as alternative Cl source without catalyst, with various catalysts and with various solvents was not successful but showing yields of not more than 32%, instead showing significant amounts of undesired by products, as documented herein under Comparative Examples 1 to 60. Only one combination, the combination of phosgene with DMF, surprisingly showed satisfying yields.
The method has the advantage of comparably high yields without the problem of generation of H3PO4 as waste.
The following abbreviations are used, if not otherwise stated:    compound of formula (1) 5-nitro-2-propylthiopyrimidine-4,6-diol, also called 5-nitro-2-propylsulfanyl-pyrimidine-4,6-diol    compound of formula (2) 4,6-dichloro-5-nitro-2-(propylthio)pyrimidine    DMF dimethyl formamide    eq equivalent    RT room temperature    MW molecular weight